It turns on an opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(someone opposed to something)
'it turns on an' is the wordplay.
'turns' is a reversal indicator.
'on' means one lot of letters go next to another.
'it' reversed gives 'ti'.
'ti' put after 'an' is 'ANTI'.
(Other definitions for anti that I've seen before include "Policy's opponent" , "Who pooh-poohs all" , "Against, informally" , "Naysayer" , "Not favouring" .)